diff --git a/decnet/mutator/ops.py b/decnet/mutator/ops.py
index efc9b296..742cb9bb 100644
--- a/decnet/mutator/ops.py
+++ b/decnet/mutator/ops.py
@@ -186,6 +186,315 @@ async def _materialise_lan_change(
         )
 
 
+def _decky_targets(decky_name: str, services: list[str]) -> list[str]:
+    """Compose service names for one decky: base + each per-decky service.
+
+    Skips ``fleet_singleton`` services — those run once fleet-wide and
+    don't have a per-decky compose entry.  Mirrors the same filter
+    applied at compose-render time
+    (:mod:`decnet.topology.compose.generate_topology_compose`).
+    """
+    from decnet.services.registry import get_service
+    targets = [decky_name]
+    for svc_name in services:
+        try:
+            svc = get_service(svc_name)
+        except KeyError:
+            # Unknown service — leave it; the compose render won't emit
+            # a fragment for it, so compose up will simply ignore the
+            # name with a clear "no such service" error.  Surface that
+            # rather than silently dropping it.
+            targets.append(f"{decky_name}-{svc_name}")
+            continue
+        if svc.fleet_singleton:
+            continue
+        targets.append(f"{decky_name}-{svc_name}")
+    return targets
+
+
+async def _live_topology_or_none(
+    repo: Any, topology_id: str,
+) -> Optional[dict[str, Any]]:
+    """Return the topology row only when it's eligible for live materialisation.
+
+    Returns None (so callers can skip with a single ``if`` check) when:
+
+    * the topology doesn't exist;
+    * status is not ``active`` or ``degraded`` (pending topologies get
+      everything materialised at deploy time);
+    * the topology is pinned to a swarm agent (cross-host live editing
+      is its own routing workstream).
+    """
+    topology = await repo.get_topology(topology_id)
+    if topology is None:
+        return None
+    if topology.get("status") not in ("active", "degraded"):
+        return None
+    if topology.get("target_host_uuid"):
+        _log.info(
+            "live decky op skipped (agent-pinned topology=%s); "
+            "next agent push will reconcile",
+            topology_id,
+        )
+        return None
+    return topology
+
+
+async def _rerender_compose(repo: Any, topology_id: str) -> None:
+    """Re-render the per-topology compose file from the current DB.
+
+    Called after each materialisation step so the file on disk matches
+    the topology rows.  Soft-fails: a render error is logged but
+    doesn't poison the DB-side mutation.
+    """
+    from decnet.engine.deployer import _topology_compose_path
+    from decnet.topology.compose import write_topology_compose
+    hydrated = await hydrate(repo, topology_id)
+    if hydrated is None:
+        return
+    try:
+        write_topology_compose(hydrated, _topology_compose_path(topology_id))
+    except Exception as exc:  # noqa: BLE001
+        _log.warning(
+            "live op: compose re-render failed topology=%s: %s",
+            topology_id, exc,
+        )
+
+
+async def _materialise_decky_spawn(
+    repo: Any, topology_id: str, decky_name: str, services: list[str],
+) -> None:
+    """compose up -d --no-deps --build for one decky (base + services).
+
+    Re-renders compose first so the file lists the new decky.  No-op
+    when the topology isn't eligible for live materialisation (see
+    :func:`_live_topology_or_none`).  Best-effort: docker failure is
+    logged, not re-raised — DB row is the source of truth.
+    """
+    if await _live_topology_or_none(repo, topology_id) is None:
+        return
+    import anyio
+    from decnet.engine.deployer import (
+        _compose_with_retry,
+        _topology_compose_path,
+    )
+    await _rerender_compose(repo, topology_id)
+    targets = _decky_targets(decky_name, services)
+    compose_path = _topology_compose_path(topology_id)
+    try:
+        await anyio.to_thread.run_sync(
+            lambda: _compose_with_retry(
+                "up", "-d", "--no-deps", "--build", *targets,
+                compose_file=compose_path,
+            ),
+        )
+    except Exception as exc:  # noqa: BLE001
+        _log.error(
+            "live add_decky: compose up failed topology=%s decky=%s: %s",
+            topology_id, decky_name, exc,
+        )
+
+
+async def _materialise_decky_remove(
+    repo: Any, topology_id: str, decky_name: str, services: list[str],
+) -> None:
+    """compose stop + rm -f for one decky's containers, then re-render."""
+    if await _live_topology_or_none(repo, topology_id) is None:
+        return
+    import anyio
+    from decnet.engine.deployer import _compose, _topology_compose_path
+
+    targets = _decky_targets(decky_name, services)
+    compose_path = _topology_compose_path(topology_id)
+    # Stop + rm BEFORE re-rendering compose; the re-rendered file no
+    # longer mentions the decky, so a stop run AFTER rendering would
+    # find no service to act on.
+    try:
+        await anyio.to_thread.run_sync(
+            lambda: _compose("stop", *targets, compose_file=compose_path),
+        )
+    except Exception as exc:  # noqa: BLE001
+        _log.warning(
+            "live remove_decky: compose stop failed topology=%s decky=%s: %s",
+            topology_id, decky_name, exc,
+        )
+    try:
+        await anyio.to_thread.run_sync(
+            lambda: _compose("rm", "-f", *targets, compose_file=compose_path),
+        )
+    except Exception as exc:  # noqa: BLE001
+        _log.warning(
+            "live remove_decky: compose rm failed topology=%s decky=%s: %s",
+            topology_id, decky_name, exc,
+        )
+    await _rerender_compose(repo, topology_id)
+
+
+async def _materialise_decky_connect(
+    repo: Any, topology_id: str,
+    decky_name: str, lan_name: str, ipv4_address: str,
+) -> None:
+    """SDK ``network.connect`` to multi-home a running base container.
+
+    Service containers share the base's netns via ``network_mode:
+    service:<base>`` (see :mod:`decnet.topology.compose`), so attaching
+    the base alone gives every service container the new interface for
+    free — we don't need to iterate.
+    """
+    if await _live_topology_or_none(repo, topology_id) is None:
+        return
+    import docker
+    from decnet.topology.compose import _container_name, _network_name
+
+    net_name = _network_name(topology_id, lan_name)
+    container_name = _container_name(topology_id, decky_name)
+    try:
+        client = docker.from_env()
+        net = client.networks.get(net_name)
+        container = client.containers.get(container_name)
+        net.connect(container, ipv4_address=ipv4_address)
+    except docker.errors.APIError as exc:
+        # Idempotency — already on the network is fine.
+        msg = str(exc).lower()
+        if "already" in msg or "endpoint" in msg and "exists" in msg:
+            _log.info(
+                "live attach_decky: %s already on network %s — skipping",
+                container_name, net_name,
+            )
+        else:
+            _log.error(
+                "live attach_decky: connect failed topology=%s decky=%s lan=%s: %s",
+                topology_id, decky_name, lan_name, exc,
+            )
+    except Exception as exc:  # noqa: BLE001
+        _log.error(
+            "live attach_decky: SDK call crashed topology=%s decky=%s lan=%s: %s",
+            topology_id, decky_name, lan_name, exc,
+        )
+    await _rerender_compose(repo, topology_id)
+
+
+async def _materialise_decky_disconnect(
+    repo: Any, topology_id: str, decky_name: str, lan_name: str,
+) -> None:
+    """SDK ``network.disconnect`` to drop a multi-home edge."""
+    if await _live_topology_or_none(repo, topology_id) is None:
+        return
+    import docker
+    from decnet.topology.compose import _container_name, _network_name
+
+    net_name = _network_name(topology_id, lan_name)
+    container_name = _container_name(topology_id, decky_name)
+    try:
+        client = docker.from_env()
+        net = client.networks.get(net_name)
+        container = client.containers.get(container_name)
+        net.disconnect(container)
+    except docker.errors.APIError as exc:
+        msg = str(exc).lower()
+        if "not connected" in msg or "no such" in msg:
+            _log.info(
+                "live detach_decky: %s already off network %s — skipping",
+                container_name, net_name,
+            )
+        else:
+            _log.error(
+                "live detach_decky: disconnect failed topology=%s decky=%s lan=%s: %s",
+                topology_id, decky_name, lan_name, exc,
+            )
+    except Exception as exc:  # noqa: BLE001
+        _log.error(
+            "live detach_decky: SDK call crashed topology=%s decky=%s lan=%s: %s",
+            topology_id, decky_name, lan_name, exc,
+        )
+    await _rerender_compose(repo, topology_id)
+
+
+async def _materialise_decky_services_diff(
+    repo: Any, topology_id: str,
+    decky_name: str,
+    added: list[str],
+    removed: list[str],
+) -> None:
+    """Add/remove per-service containers without touching siblings.
+
+    Mirrors :mod:`decnet.engine.services_live`'s up/down pattern but
+    without coupling the mutator to that module — service mutations
+    routed via the mutator queue publish ``mutation.applied`` while the
+    direct API publishes ``decky.<name>.service_added``; they share
+    machinery, not control flow.
+    """
+    if not added and not removed:
+        return
+    if await _live_topology_or_none(repo, topology_id) is None:
+        return
+    import anyio
+    from decnet.engine.deployer import (
+        _compose, _compose_with_retry, _topology_compose_path,
+    )
+    await _rerender_compose(repo, topology_id)
+    compose_path = _topology_compose_path(topology_id)
+    add_targets = _decky_targets(decky_name, list(added))[1:]  # drop the base
+    if add_targets:
+        try:
+            await anyio.to_thread.run_sync(
+                lambda: _compose_with_retry(
+                    "up", "-d", "--no-deps", "--build", *add_targets,
+                    compose_file=compose_path,
+                ),
+            )
+        except Exception as exc:  # noqa: BLE001
+            _log.error(
+                "live update_decky add: compose up failed topology=%s decky=%s: %s",
+                topology_id, decky_name, exc,
+            )
+    rm_targets = _decky_targets(decky_name, list(removed))[1:]
+    for action_name, args in (("stop", ("stop",)), ("rm", ("rm", "-f"))):
+        if not rm_targets:
+            break
+        try:
+            await anyio.to_thread.run_sync(
+                lambda args=args: _compose(*args, *rm_targets, compose_file=compose_path),
+            )
+        except Exception as exc:  # noqa: BLE001
+            _log.warning(
+                "live update_decky %s failed topology=%s decky=%s: %s",
+                action_name, topology_id, decky_name, exc,
+            )
+
+
+async def _materialise_decky_recreate_base(
+    repo: Any, topology_id: str, decky_name: str,
+) -> None:
+    """Force-recreate just the base container (used for forwards_l3 flips).
+
+    DESTRUCTIVE: kills any in-container state on the base.  Service
+    containers re-attach via ``network_mode: service:<base>`` after the
+    base is rebuilt.  Caller is responsible for gating this on an
+    explicit operator-supplied ``force=true`` flag.
+    """
+    if await _live_topology_or_none(repo, topology_id) is None:
+        return
+    import anyio
+    from decnet.engine.deployer import (
+        _compose_with_retry, _topology_compose_path,
+    )
+    await _rerender_compose(repo, topology_id)
+    compose_path = _topology_compose_path(topology_id)
+    try:
+        await anyio.to_thread.run_sync(
+            lambda: _compose_with_retry(
+                "up", "-d", "--no-deps", "--force-recreate", decky_name,
+                compose_file=compose_path,
+            ),
+        )
+    except Exception as exc:  # noqa: BLE001
+        _log.error(
+            "live update_decky recreate_base failed topology=%s decky=%s: %s",
+            topology_id, decky_name, exc,
+        )
+
+
 # ------------------------------------------------------------------- ops
 
 
@@ -308,11 +617,12 @@ async def apply_add_decky(
     if forwards_l3:
         decky_config["forwards_l3"] = True
 
+    services_list = list(payload.get("services", []))
     decky_uuid = await repo.add_topology_decky(
         {
             "topology_id": topology_id,
             "name": name,
-            "services": list(payload.get("services", [])),
+            "services": services_list,
             "decky_config": decky_config,
             "x": payload.get("x"),
             "y": payload.get("y"),
@@ -327,6 +637,10 @@ async def apply_add_decky(
             "forwards_l3": forwards_l3,
         }
     )
+    # Live materialisation: spawn the new decky's containers without
+    # touching siblings.  Skips on pending / agent-pinned topologies —
+    # see _live_topology_or_none.
+    await _materialise_decky_spawn(repo, topology_id, name, services_list)
     await _assert_valid_after(repo, topology_id)